The present invention relates to digital signal processing, and more particularly to digital filters and methods.
Recorded music playback typically includes equalization filtering which can emphasize or attenuate frequency bands in order to improve the sound. A set of tunable equalization filters, each of which modifies a frequency band while leaving frequencies outside of the band unchanged, can be cascaded to provide graphic equalization filtering with control of the gains in the individual frequency bands; FIG. 4 shows generic functional blocks. A graphic equalizer may include a display of the power (gain) in each of the frequency bands; this allows easy user adjustments of the gains.
Regalia et al, Tunable Digital Frequency Response Equalization Filters, 35 IEEE Trans. Acoustics, Speech, and Signal Processing 118 (1987), Fontana et al, A Digtial Bandpass/Bandstop Complementary Equalization Filter with Independent Tuning Characteristics, 10 IEEE Signal Processing Lett. 119 (2003), and Bristow-Johnson, The Equivalence of Various Methods of Computing Biquad Coefficients for Audio Parametric Equalizers, Proc. Audio Eng. Soc. Conv. (November 1994) describe designs for individual tunable equalizer filters: first-order filters can be used for the high frequency and low frequency ends (shelf filters), and second-order filters can be used for the center bands (peak/notch filters). These filters provide single parameter (gain) control for each band and are cascaded to form a graphic (parametric) equalizer.
However, there is a problem of gain leakage among the adjacent bands in cascades of such individual equalization filters. Indeed, the magnitude response of one equalization filter will affect the responses of the other filters, and this “leak gain” affects the overall cascaded filter system response.